How many neutrons are present in an atom of Sodium-20?

A. 12

B. 11

C. 10

D. 9

Mass number = atomic number (number of protons) + number of neutrons.

Sodium-20 - 20 is a mass number

Atomic number you need to find from the Periodic table. It is the ordinal number of the element.

Atomic number of Sodium (Na) is 11.

20=11 + number of protons

Number of neutrons is 20-11=9.

Correct answer D. 9.